Question

When boiling a saturated liquid, one must be careful while increasing the heat flux to avoid burnout. Burnout occurs when the boiling transitions from boiling. (a) convection to nucleate (b) convection to film (c) film to nucleate (d) nucleate to film (e) none of them

Short answer

boiling.

Step-by-step solution

Understanding the boiling process.

Boiling is a heat transfer process that involves phase change from liquid to vapor, often due to the application of heat. There are two dominant types of boiling, nucleate boiling and film boiling. Nucleate boiling occurs when vapor bubbles form at the solid-liquid interface due to localized heating. This is considered the most efficient way of boiling as the heat transfer coefficient is high during this process. Film boiling, on the other hand, occurs when a stable vapor film covers the solid-liquid interface, which causes a sharp reduction in heat transfer efficiency. The heat is transferred mainly by radiation, and it is less efficient than nucleate boiling.

Analyzing the given options.

Let's go through each option and analyze if it corresponds to the burnout phenomenon: (a) Convection to nucleate boiling: This transition would represent an increase in heat transfer efficiency, which is not associated with burnout. (b) Convection to film boiling: This transition would represent a decrease in heat transfer efficiency. When the heat flux is increased beyond a certain point during nucleate boiling, the bubbles formed can merge into a continuous vapor film, leading to a decrease in heat transfer efficiency and potential burnout. (c) Film to nucleate boiling: This transition would represent an increase in heat transfer efficiency, which is not associated with burnout. (d) Nucleate to film boiling: Similar to option (b), this transition represents a decrease in heat transfer efficiency, leading to the possible occurrence of burnout. (e) None of them: Considering options (b) and (d), this option is incorrect because burnout can occur during certain transitions.

Selecting the correct answer.

Since burnout occurs when the boiling transitions from nucleate boiling to film boiling due to a significant decrease in heat transfer efficiency, the correct answer is: (d) nucleate to film

Key concepts

Nucleate Boiling

Nucleate boiling is a fascinating and highly efficient heat transfer process. It occurs when small vapor bubbles form at discrete points on a heated surface in contact with a liquid. These points, or nucleation sites, facilitate bubble formation due to localized heat, which causes the liquid to turn into vapor. During nucleate boiling, the heat transfer coefficient is remarkably high because the bubbles rapidly rise, detach, and are replaced by cooler liquid. This continuous cycle maximizes the efficiency of heat transference. The process is common in many everyday applications, such as in cooking and industrial heating systems, as it allows for superior heat management. It's important to note that the occurrence of nucleate boiling is dependent on several factors like pressure, surface condition, and the nature of the fluid.

Film Boiling

In film boiling, the dynamics of heat transfer change significantly compared to nucleate boiling. This mode of boiling occurs when the surface temperature is much higher than the boiling point of the liquid, leading to the formation of a stable vapor film between the liquid and the heated surface. The presence of this vapor film acts as an insulating layer, drastically reducing the heat transfer efficiency. As the heat is primarily transferred by radiation and conduction through the vapor layer, the overall process is far less efficient than nucleate boiling.

• Film boiling often occurs at extremely high temperatures.
• It is visually characterized by a rolling motion as the vapor film covers the surface.
• The transition to this state from nucleate boiling can signal the onset of thermal discomfort or burnout.

In many industrial settings, avoiding film boiling is crucial since it can lead to overheating and potential damage to equipment.

Heat Flux

Heat flux is a measure of the rate of heat energy transfer across a given surface area. It plays a crucial role in boiling heat transfer processes. Heat flux is vital for understanding how efficiently a system transfers thermal energy. In boiling operations, controlling heat flux is key:

• A low heat flux may only sustain natural convection, where the slower movement of fluid hinders effective heat transfer.
• An optimal increase in heat flux facilitates nucleate boiling, enhancing energy efficiency and heat dissipation.
• Excessive heat flux can lead to burnout, transitioning the system into film boiling, reducing the system's heat transfer capability.

Engineers and scientists frequently monitor and adjust heat flux to optimize the boiling process, prevent burnout, and ensure system integrity and safety.

Burnout Phenomenon

The burnout phenomenon is an adverse situation in boiling processes, denoting a critical level in heat flux. It is a significant concern when boiling a saturated liquid. During nucleate boiling, if the heat flux increases excessively, it can disrupt the regular boiling cycle. As the heat flux reaches a peak, the vapor bubbles can coalesce into a continuous vapor film, causing a transition from nucleate to film boiling. This shift is detrimental because it leads to:

• A severe drop in heat transfer efficiency.
• Potential mechanical failure as surface temperatures rise dramatically.
• Increased risk of damage to both equipment and surrounding areas.

Understanding and preventing burnout is essential for maintaining efficient heat transfer and safeguarding equipment in industrial and domestic applications. Monitoring and controlling heat inputs are vital steps toward mitigating this phenomenon.